The present invention relates to a pulse magnetron designed for pulsing to generate microwaves. More particularly, the present invention relates to a pulse magnetron which has a construction for effectively attenuating generation of spurious radiation.
Such a magnetron includes, as shown in FIG. 7, a number of vanes 12 mounted radially on the inner wall of a cylindrical anode shell 11 with a cavity provided between any two adjacent vanes and the anode shell 11 and connected alternatively by straps 14 for stabilizing the oscillation in a n mode which all constitute an anode 1. As a cathode 2 is located at the center of the anode 1, the anode shell 11 has pole pieces 3 mounted to both axial ends thereof for applying a magnetic field substantially in parallel to the surface of the cathode 2 across an interaction space 4 between the inner side (at the inner end of the vanes 12) of the anode 1 and the outer side of the cathode 2. This causes electrons from the cathode 2 to make circular orbits by the right-angle force of the magnetic field in the interaction space 4 thus introducing energy to the resonant cavities for oscillation. The magnetron is commonly used in a radar system and energized with an anode voltage for pulsing operation.
In recent years, as a variety of microwave generators have been in use, their generation of spurious radiation is strictly controlled under relevant regulations. It is also a drawback of the pulse magnetron to develop spurious radiation at frequencies close to the fundamental oscillation frequency. When the magnetron used in a radar system is pulsed, its oscillation output level has a number of other lobes at sidebands in addition to the main lobe in the spectrum shown in FIG. 8. The spectrum is determined by the pulse width provided for actuating the pulse magnetron is not narrower than a spectrum of a Fourier analysis based on a oscillating output waveform. Inversely in general, the spectrum may be wider than its theoretical size due to various causes. Also, the shape of the spectrum is not linearly symmetrical about the fundamental oscillation frequency but may be biased as having a noticeable lobe profile (P) at one sideband, shown in FIG. 8, which causes spurious radiation.
One of the causes for creating faults in the spectrum such as an unsymmetrical shape or a noticeable lobe at the sideband may be oscillation off the predetermined operating timing at the rise in the pulse magnetron. When the anode voltage is gradually increased, the oscillation of the pulse magnetron will start at a current about 5 to 10% lower than its rated level. The output is thus 40 to 50 dB lower than the rated level as the oscillation is made at a frequency lower than the fundamental oscillation frequency. Since the pulse magnetron having the above described operating characteristics is pulsed, it is timed at such a lower current range with each pulse rise in the lower side of the fundamental frequency and its output is 40 to 50 dB lower than the rated level. As the result, the frequency spectrum will be unsymmetrical having a noticeable profile of −40 to −50 dBc (decibels) at one sideband.
It is hence known that the spurious radiation is caused by non-uniformity in the magnetic field at the interaction space between the anode and the cathode and thus variation in the relationship between the magnetic flux density and the electric field intensity. Then tentatively, the generation of noise can be attenuated by the vanes modified with its axial ends projecting more than the center in the axial direction.